1. Field of the Invention
The present invention relates to a method of fabricating a DRAM. More particularly, the invention relates to a method of fabricating self-aligned contacts between conductor lines.
2. Description of the Prior Art
A process of producing the DRAM main purpose of self-alignment is that the isolation effect for the isolating layer being not reduced due to subsequent etching steps.
FIGS. 1A to 1B show cross-sectional views of a process for producing a non self-aligned contact.
Referring to FIG. 1A, a process for producing a non self-aligned contact includes forming conductor lines 102 on a substrate 100, depositing a dielectric layer 104 having a flat upper surface on the substrate 100 and conductor line 102; then defining a contact hole 106 into the dielectric layer 104 between two conductor lines 102 by etching so as to expose and connect the substrate 100.
Next, referring to FIG. 1B, an isolating spacer 108 is formed on the sidewall of the contact holes 106. Finally, a conductive material is deposited to form a conductive plug 110.
With increasing integrity, the distance between the conductor lines is shortened. In the above mentioned process for producing contacts, it may result in exposure of the conductor lines to the contact hole because of non self-alignment when a dielectric layer is etched to form contact holes.
Two processes for producing a self-aligned contact to improve the disadvantages caused from the above method are known in the art. One of them is showed in FIGS. 2A-2C, the other in FIGS. 2A, 2D-2F.
Referring to FIG. 2A, a general process for producing a self-aligned contact includes forming conductor lines 202 on a substrate 200, depositing a cap layer 204 on the leads 202, then forming an isolating layer 206 on the substrate 200 and cap layer 204.
Next, referring to FIG. 2B, etching is carried out so as to allow the cap layer 204 to be exposed, and an isolating spacer 206a is formed on both sidewalls of the conductor line 202.
Finally, referring to FIG. 2C, a dielectric layer 208 with a flat upper surface is deposited on the substrate 200 and cap layer 204. A contact hole which will be connected with the substrate 200 is defined between two conductor lines 202 and formed by etching part of the dielectric layer 208. Finally, a conductive material is deposited in the contact hole to form a conductive plug 212.
With the process for producing a contact, a part of the cap layer and spacer will be etched off to be thinner. It will result in the poor isolation characteristics between conductor lines and plugs.
Referring to FIG. 2A, another process for producing self-aligned contact is disclosed. Conductor lines 202 are patterned on a substrate 202. A cap layer 204 is deposited on the leads 202. Then, an isolating layer 206 is deposited on the cap layer 204 and substrate 200.
Referring to FIG. 2E, a contact hole 210 which will be connected with the substrate 200 is defined between two conductor lines 202 and formed by etching a part of the dielectric layer 208.
Referring to FIG. 2F, a conductive material is deposited in the contact hole to form a conductive plug 212.
When the dielectric layer was etched by this method, the isolating layer in the bottom of the contact hole will be etched together with the dielectric layer. The bottom of contact hole will have residues if existing non-complete etching on bottom isolating layer. It will result in increased resistance of the contact between the contact hole and the substrate.
The invention thus provides a process for producing a self-aligned contact, which improves the disadvantages of the prior art, such as poor isolating characteristics between contact holes and conductor lines and non complete etching on the bottom area between the contact hole and substrate.
The process according to the present invention comprises the steps of forming conductor lines on a substrate, forming an etching stop layer on the conductor lines by depositing, then forming a sacrificed oxide layer, after the structure of the conductor lines is defined, forming a spacer on each side of the structure, removing the sacrificed oxide layer, allowing the spacer to protrude in the form of horn. Next, a dielectric layer having a flat upper surface is deposited on the substrate and the conductor lines, a contact hole being formed between the structure of the conductor lines so as to connect the substrate, and a conductive material then being filled in the contact hole and back-etched to form a plug.
The main advantage of the present invention is that: with the structure of spacer, a stack structure of a conductor line layer, an etching stop layer and a sacrificed oxide layer, the spacer subsequently formed is pulled up, then protruding in the form of horn by removing the sacrificed oxide layer.
Because the etchant with higher etching selectivity for the dielectric layer to silicon nitride is used during forming the contact hole by removing part of the dielectric layer, the tip of the horn-shaped silicon nitride spacer is etched less. It can avoid the spacer at the top outside the structure of conductor lines to be etched off to cause the problem of poor isolating characteristics and exposure of conductor lines.
Additionally, according to the present invention, the spacer is very important during forming the contact hole by removing the dielectric layer. It can, improve the isolation between the conductor lines and the conductive plug. Furthermore, the problem of non-complete etching of the dielectric layer in the bottom of the contact can be also improved.
Another advantage of the present invention is to improve the process window. Because the spacer can not be etched readily during forming the contact hole by etching the dielectric layer, the limit to the time of etching has a broader range, achieving an improvable process window.
There is lower contact resistance between the contact hole and the substrate, because the contact area between the contact hole and substrate is etched more completely.